Anthropic in 5 Days: Sonnet 5 as the Default, the Return of Fable 5, and New Security Measures

In just five days, Anthropic has accomplished quite a bit: a new default model, the return of the previously disabled Fable 5, and a package of safety initiatives. Claude Code users noticed that Sonnet 5 has appeared in the header without the need for manual switching—and this isn’t a bug, but a deliberate decision.

Sonnet 5 — The New Default

The model is set as the default on all plans. According to the reported data, Sonnet 5 scores 80.4% on Terminal-Bench, outperforming Opus 4.8, while costing three times less. At first glance, this represents a significant shift in the price-to-performance ratio.

However, there’s a catch: according to the author’s observations, the new tokenizer cancels out 30–40% of the claimed savings. In other words, the actual benefit from the price reduction turns out to be significantly more modest than the marketing figures suggest.

The Return of Fable 5

After 19 days “on hold”, Fable 5 is available again. Along with its return, Anthropic introduced several security mechanisms:

• cyber safeguards — protective restrictions for cyber scenarios;
• jailbreak severity framework — a system for assessing the severity of attempts to bypass security;
• integration with HackerOne for bug hunters searching for vulnerabilities.

What Happens in Practice

The author tested Sonnet 5 on their work tasks and shares their observations: the model does indeed behave differently, but the savings on tokens are offset by the new tokenizer. The question of where exactly the real benefit has “moved” remains open.

Source: All Articles / Artificial Intelligence / Habr